Cobalt resistance recovery by hydrogen anneal

ABSTRACT

Resistance increase in Cobalt interconnects due to nitridation occurring during removal of surface oxide from Cobalt interconnects and deposition of Nitrogen-containing film on Cobalt interconnects is solved by a Hydrogen thermal anneal or plasma treatment. Removal of the Nitrogen is through a thin overlying layer which may be a dielectric barrier layer or an etch stop layer.

BACKGROUND

Technical Field

The disclosure concerns a method of forming layered structures havingconductive Cobalt interconnects for interlayer connectivity in anintegrated circuit such as a complementary metal oxide semiconductor(CMOS) structure.

Background Discussion

As critical dimension (CD) is reduced for CMOS devices, line resistanceof the conductive interconnects increases. The conductive interconnectsare typically Copper. In order to address the problem of increasing lineresistance, the conductive interconnects may be formed of Cobalt insteadof Copper.

Following chemical mechanical polishing (CMP), exposed surfaces of theCobalt interconnects tend to form an overlying thin Cobalt oxide layer,which must be removed. One method for removing the thin Cobalt oxidelayer is to treat the structure with an ammonia plasma (a plasma formedof NH3). This treatment can enhance time dependent dielectric breakdown(TDDB) behavior of the structure. It is a challenge for one to removethe Cobalt oxide layer without damaging the under layer.

SUMMARY

In accordance with a first aspect, a method of processing a workpiececomprises forming on the workpiece a dielectric layer and an interlayerinterconnect extending through the dielectric layer, removing oxide froman exposed surface of the interlayer interconnect by treating theworkpiece in a plasma formed of a Nitrogen-containing gas, anddepositing on the interlayer interconnect a dielectric barrier layer ofa thickness less than a threshold thickness. The method furthercomprises reducing resistance of the interlayer interconnect by removingNitrogen from the interlayer interconnect through the dielectric barrierlayer and increasing thickness of the dielectric barrier layer above thethreshold thickness.

In one embodiment, the Nitrogen-containing gas comprises ammonia. In oneembodiment, the interlayer interconnect comprises Cobalt.

In one embodiment, the threshold thickness does not exceed 50 Angstroms,and may be about 20 Angstroms.

In one embodiment, the removing Nitrogen from the interlayerinterconnect comprises exposing the workpiece to a Hydrogen plasma,radicals or Hydrogen thermal anneal.

In one embodiment, the dielectric barrier layer comprises Silicon andone or more of the following: Carbon, Oxygen, Nitrogen.

In one embodiment, the threshold thickness is sufficiently small topermit removal of Nitrogen through the dielectric barrier layer by aHydrogen plasma, radicals or Hydrogen thermal anneal.

In accordance with a second aspect, a method of processing a workpiececomprises forming on the workpiece a dielectric layer and an interlayerinterconnect extending through the dielectric layer and removing oxidefrom an exposed surface of the interlayer interconnect by treating theworkpiece in a plasma formed from a Nitrogen-containing gas or Hydrogenplasma, radicals or thermal anneal. The method further comprisesdepositing on the interlayer interconnect an etch stop layer of athickness less than a threshold thickness, reducing resistance of theinterlayer interconnect by removing Nitrogen from the interlayerinterconnect through the etch stop layer, and increasing thickness ofthe etch stop layer above the threshold thickness.

In one embodiment, the interlayer interconnect comprises Cobalt.

In one embodiment, the threshold thickness is less than 50 Angstroms ormay be about 20 Angstroms.

In one embodiment, the removing Nitrogen from the interlayerinterconnect comprises exposing the workpiece to a Hydrogen plasma,radicals or Hydrogen thermal anneal.

In one embodiment, the etch stop layer comprises a Nitrogen-containingmaterial such as AlN.

In one embodiment, the Nitrogen-containing gas comprises ammonia.

In one embodiment, the threshold thickness is sufficiently small topermit removal of Nitrogen through the etch stop layer by a Hydrogenplasma, radicals or Hydrogen thermal anneal.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the presentinvention are attained can be understood in detail, a more particulardescription of the invention, briefly summarized above, may be had byreference to the embodiments thereof which are illustrated in theappended drawings. It is to be appreciated that certain well knownprocesses are not discussed herein in order to not obscure theinvention.

FIGS. 1A, 1B, 1C, 1D and 1E depict successive side views of anintegrated circuit structure, the successive side views corresponding toa sequence of process operations.

FIG. 2 is a block flow diagram of the sequence of process operationscorresponding to the succession of FIG. 1A through FIG. 1E.

FIGS. 3A, 3B, 3C, 3D and 3E depict successive side views of anintegrated circuit structure, the successive side views corresponding toa sequence of process operations.

FIG. 4 is a block flow diagram of the sequence of process operationscorresponding to the succession of FIG. 3A through FIG. 3E.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the appendeddrawings illustrate only exemplary embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

As previously mentioned, removing the Cobalt oxide layer withoutdamaging an under layer is challenging. One problem we have discoveredwith Cobalt interconnects is that exposure to Nitrogen orNitrogen-containing substances, such as an ammonia plasma, causes theresistance of the Cobalt interconnect to increase. We believe this isdue to nitridation of the Cobalt interconnects. This increase can besignificant, e.g., about 5% to 25%, depending upon structure size anddevice density. Therefore, one problem is how to avoid increasedresistance due to nitridation.

In some cases, a dielectric barrier layer is deposited on the Cobaltinterconnects after Cobalt oxide removal. Such a dielectric barrierlayer contains Silicon in combination with other materials such asCarbon, Oxygen and/or Nitrogen. Contact of the Cobalt interconnect withthe Silicon-containing barrier layer causes silicidation of the Cobaltinterconnect. Such silicidation increases the line resistance of theCobalt interconnect. Therefore, a second problem is how to provide aSilicon-containing barrier layer on top of the Cobalt interconnectwithout causing a resistance increase due to silicidation of the Cobaltinterconnect by Silicon from the etch stop layer.

In other cases, an etch stop layer is deposited over the Cobaltinterconnects after Cobalt oxide removal. The etch stop layer istypically a Nitrogen-containing material such as Aluminum nitride (AlN)and is left in place at least until completion of a subsequent etchoperation in the process. Even if a non-ammonia process is used forCobalt oxide removal (e.g. Hydrogen plasma, radicals or gas anneal),contact of the nitrogen-containing etch stop layer with the Cobaltinterconnect leads to nitridation of the Cobalt interconnect, whichincreases line resistance of the Cobalt interconnects. Therefore, athird problem is how to provide a Nitrogen-containing etch stop layer ontop of the Cobalt interconnect without causing a resistance increase dueto nitridation of the Cobalt interconnect by Nitrogen from the etch stoplayer.

Referring to FIG. 1A and block 200 of FIG. 2, a dielectric layer 90 isone of plural layers of a multilayer semiconductor structure formed on aworkpiece 92 such as a semiconductor wafer. The dielectric layer 90 mayinclude a bottom dielectric layer 100 of a material having a lowdielectric constant. A Cobalt interconnect 104 extends from the bottomdielectric layer 100 through the dielectric layer 90 to top surface 90 aof the dielectric layer 90. The structure includes a large number ofCobalt interconnects, only one of which is illustrated in the drawings.Thus, the Cobalt interconnect 104 is one of plural interconnectsextending through the dielectric layer 90. The workpiece 92 is treatedby chemical mechanical polishing, which leaves top surface 104 a of theCobalt interconnect 104 exposed. The top surface 104 a oxidizes uponexposure to form a Cobalt oxide layer 106. The workpiece 92 is placed ina plasma reactor chamber 107 (indicated in dashed line) where it mayremain during the rest of the process of FIG. 2. Alternatively,different operations of the process can be done in different chambers,not necessarily in one chamber.

As depicted in FIG. 1B, the Cobalt oxide layer 106 is removed by anoxide reduction process that employs an ammonia plasma (block 205 ofFIG. 2). Some Nitrogen from the ammonia plasma accumulates below the topsurface 104 a and forms a Nitrogen-containing zone 108 in the Cobaltinterconnect 104. This may be referred to as nitridation. The presenceof the Nitrogen in the Cobalt interconnect 104 increases the electricalresistance of the Cobalt interconnect.

The Nitrogen-containing zone 108 is resistant or immune to silicidation,and is left in place temporarily to prevent silicidation duringsubsequent deposition of a Silicon-containing dielectric barrier layer,as will now be described.

As shown in FIG. 1C, a dielectric barrier layer 110 is deposited (block210 of FIG. 2). This deposition may be performed using a plasma enhancedchemical vapor deposition (PECVD) process or using a physical vapordeposition (PVD) process, for example. The dielectric barrier layer 110is thin (about 20 Angstroms). The dielectric barrier layer 110 may be aSilicon-containing material including other materials such as Carbon,Oxygen and/or Nitrogen and/or may be characterized by a low dielectricconstant.

As shown in FIG. 1D, the Nitrogen in the Cobalt interconnect 104 isremoved by Hydrogen plasma, radicals or Hydrogen thermal anneal thatemploys Hydrogen in the chamber (block 215 of FIG. 2). The dielectricbarrier layer 110 is sufficiently thin (e.g., 20 Angstroms, or less than50 Angstroms) for the Nitrogen to be removed through the dielectricbarrier layer 110 from the Cobalt interconnect 104 by Hydrogen plasma,radicals or Hydrogen thermal anneal. Such removal of the Nitrogen undoesnitridation that would otherwise increase resistance of the Cobaltinterconnect 104. We have found that this treatment by Hydrogen returnsthe resistance of the Cobalt interconnects to the original (lesser)value that it had prior to the exposure of the Cobalt interconnects tothe ammonia plasma.

Thereafter, as shown in FIG. 1E, the thickness of the dielectric barrierlayer 110 may be increased to a desired thickness (e.g., 100 Angstroms)by deposition of additional dielectric barrier layer material 112 (block220 of FIG. 2). This deposition may be performed using a plasma enhancedchemical vapor deposition (PECVD) process or using a physical vapordeposition (PVD) process, for example.

A second embodiment will now be described. Referring to FIG. 3A and toblock 400 of FIG. 4, a dielectric layer 190 is one of plural layers of amultilayer semiconductor structure formed on a workpiece 192 such as asemiconductor wafer. The dielectric layer 190 may include a bottomdielectric layer 300 of low dielectric constant. A Cobalt interconnect304 extends from the bottom dielectric layer 300 through the dielectriclayer 190 to top surface 190 a of the dielectric layer 190. Thestructure includes a large number of Cobalt interconnects, only one ofwhich is illustrated in the drawings. Thus, the Cobalt interconnect 304is one of plural interconnects extending through the dielectric layer190. The workpiece 192 is treated by chemical mechanical polishing,which leaves top surface 304 a of the Cobalt interconnect 304 exposed.The top surface 304 a oxidizes upon exposure to form a Cobalt oxidelayer 306. The workpiece 192 is placed in a plasma reactor chamber 307(indicated in dashed line) and may remain there during the rest of theprocess of FIG. 4. Alternatively, different operations of the processmay be performed in different chambers.

As depicted in FIG. 3B, the Cobalt oxide layer 306 is removed bygenerating an ammonia plasma in the reactor chamber (block 405 of FIG.4). Alternatively, the Cobalt oxide removal may be performed in anactive pre-clean process that employs active species such as (but notlimited to) Hydrogen radicals. If the Cobalt oxide is removed using anammonia plasma, then Nitrogen from the ammonia plasma accumulates belowthe top surface 304 a to form a nitrogen-containing zone 308 in theCobalt interconnect 304. The presence of the Nitrogen in the Cobaltinterconnect 304 increases the electrical resistance of the Cobaltinterconnect.

As shown in FIG. 3C, an etch stop layer 310 is deposited (block 410 ofFIG. 4). The etch stop layer 310 is thin (about 20 Angstroms). The etchstop layer 310 may be a Nitrogen-containing material such as Aluminumnitride (AlN), and therefore its deposition contributes to nitridationof the Cobalt interconnect 304. This is a significant feature where anactive pre-clean process was used to perform the Cobalt oxide removal,because the active pre-clean process does not provide nitridation of theCobalt interconnect. In such a case, nitridation is provided by the AlNetch stop layer deposition. Deposition of the AlN etch stop layer 310may be performed in a CVD process or in a PECVD process or in a physicalvapor deposition (PVD) process, for example.

As shown in FIG. 3D, Nitrogen in the Cobalt interconnect (e.g., in theNitrogen-containing zone 308) is removed by Hydrogen plasma, radicals orHydrogen thermal anneal in the chamber (block 415 of FIG. 4) using aHydrogen gas (H2). The etch stop layer 310 is sufficiently thin (e.g.,20 Angstroms, or less than 50 Angstroms) for the Nitrogen to be removedfrom the Cobalt interconnect 304 by the Hydrogen plasma, radicals orHydrogen thermal anneal through the etch stop layer 310. This removal ofthe Nitrogen undoes nitridation that would otherwise increase electricalresistance of the Cobalt interconnect 304. We have found that thistreatment by Hydrogen returns the resistance of the Cobalt interconnectsto the original (lower) value that it had prior to the exposure of theCobalt interconnects to Cobalt nitridation by ammonia plasma orNitrogen-containing etch stop.

Thereafter, as shown in FIG. 3E, the thickness of the etch stop layer310 may be increased to a desired thickness (e.g., 100 Angstroms) bydeposition of additional etch stop material 312 (block 420 of FIG. 4) onthe thin etch stop layer 310. The thin etch stop layer 310 protects theCobalt interconnect from Nitrogen in the additional etch stop material312. This deposition may be performed by CVD or PECVD or PVD process,for example.

Advantages:

Embodiments described above solve the problem of resistance increase inCobalt interconnects by nitridation and silicidation. Nitridation occursduring removal of surface oxide from the Cobalt interconnects by anammonia plasma. Nitridation is removed by a Hydrogen treatment through adielectric layer, and silicidation is prevented. The nitridation of theCobalt interconnects is exploited by temporarily leaving the nitride inplace while depositing a Silicon-containing layer. The nitride blockssilicidation of the Cobalt interconnects during deposition of theSilicon-containing layer. The Nitrogen is removed through the initialSilicon-containing layer, which is sufficiently thin to enable Hydrogento draw out the Nitrogen in the Cobalt interconnects through the initialSilicon-containing layer. Thereafter, the thickness of theSilicon-containing layer may be increased by further deposition of theSilicon-containing material without silicidation of the Cobaltinterconnects, because the initial thin layer of Silicon-containingmaterial protects the Cobalt interconnects. The plasma reactor chamber107 may be an integrated tool capable of performing each one of theprocesses or operations referred to above without removing the workpiecefrom the integrated tool 107. In one embodiment, an integrated toolperforms the foregoing operations in the same chamber. In anotherembodiment, an integrated tool performs different operations indifferent chambers. In a further embodiment, different operations areperformed in different tools.

Embodiments described above solve the problem of resistance increase inCobalt interconnects by nitridation from a Nitrogen-containing etch stoplayer (e.g., AlN). In this case, nitridation occurs by the exposure ofCobalt to a Nitrogen-containing film. Nitridation is removed by aHydrogen treatment through an initial AlN layer. The initial AlN layeris sufficiently thin to enable the Hydrogen to draw out the Nitrogen inthe Cobalt interconnects through the initial AlN layer. Thereafter, thethickness of the AlN layer may be increased by further deposition of AlNmaterial without nitridation of the Cobalt interconnects, because theinitial thin AlN layer protects the Cobalt interconnects. The plasmareactor chamber 107 may be an integrated tool capable of performing eachone of the processes or operations referred to above without removingthe workpiece from the integrated tool 107. In one embodiment, anintegrated tool performs the foregoing operations in the same chamber.In another embodiment, an integrated tool performs different operationsin different chambers. In a further embodiment, different operations areperformed in different tools.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method of processing a workpiece, comprising:forming on the workpiece a dielectric layer and an interlayer conductiveinterconnect extending through said dielectric layer; removing oxidefrom an exposed surface of said interlayer conductive interconnect bytreating said workpiece in a plasma formed of a Nitrogen-containing gas,to form a Nitrogen-containing surface zone in said interlayer conductiveinterconnect and leaving in place said Nitrogen-containing surface zoneas a silicidation-preventing layer; depositing on said interlayerconductive interconnect a Silicon-containing dielectric barrier layer ofa thickness less than a threshold thickness; reducing electricalresistance of said interlayer conductive interconnect by removingNitrogen from said interlayer conductive interconnect through saiddielectric barrier layer; and increasing thickness of said dielectricbarrier layer above said threshold thickness.
 2. The method of claim 1wherein said Nitrogen-containing gas comprises ammonia.
 3. The method ofclaim 1 wherein said interlayer conductive interconnect comprisesCobalt.
 4. The method of claim 1 wherein said threshold thickness isabout 20 Angstroms.
 5. The method of claim 1 wherein said removingNitrogen from said interlayer conductive interconnect comprises exposingsaid workpiece to a Hydrogen plasma, radicals or Hydrogen thermalanneal.
 6. The method of claim 5 wherein said dielectric barrier layerfurther comprises one or more of the following: Carbon, Oxygen,Nitrogen.
 7. The method of claim 1 wherein said threshold thickness issufficiently small to permit removal of Nitrogen through said dielectricbarrier layer by a Hydrogen plasma, radicals or Hydrogen thermal anneal.8. A method of processing a workpiece, comprising: forming on theworkpiece a dielectric layer and an interlayer conductive interconnectextending through said dielectric layer; removing oxide from an exposedsurface of said interlayer conductive interconnect; depositing on saidinterlayer conductive interconnect an etch stop layer of a thicknessless than a threshold thickness; reducing electrical resistance of saidinterlayer conductive interconnect by removing Nitrogen from saidinterlayer conductive interconnect through said etch stop layer; andincreasing thickness of said etch stop layer above said thresholdthickness.
 9. The method of claim 8 wherein said interlayer conductiveinterconnect comprises Cobalt.
 10. The method of claim 8 wherein saidthreshold thickness is about 20 Angstroms.
 11. The method of claim 8wherein said threshold thickness is sufficiently small to permit removalof Nitrogen through said etch stop layer by a Hydrogen plasma, radicalsor Hydrogen thermal anneal.
 12. The method of claim 8 wherein said etchstop layer comprises a Nitrogen-containing material.
 13. The method ofclaim 12 wherein said etch stop layer comprises AlN.
 14. The method ofclaim 8 wherein said removing Nitrogen from said interlayer conductiveinterconnect comprises treating said interlayer conductive interconnectwith one of a Hydrogen plasma, radicals or Hydrogen thermal anneal. 15.A method of processing a workpiece, comprising: forming on the workpiecea dielectric layer and an interlayer conductive interconnect extendingthrough said dielectric layer; removing oxide from an exposed surface ofsaid interlayer conductive interconnect; depositing on said interlayerconductive interconnect a deposition layer of a thickness less than athreshold thickness, said deposition layer comprising one of adielectric barrier layer or an etch stop layer; reducing electricalresistance of said interlayer conductive interconnect by removingNitrogen from said interlayer conductive interconnect through saiddeposition layer; and increasing thickness of said deposition layerabove said threshold thickness.
 16. The method of claim 15 wherein saidinterlayer conductive interconnect comprises Cobalt.
 17. The method ofclaim 15 wherein said removing Nitrogen from said interlayer conductiveinterconnect comprises treating said interlayer conductive interconnectwith one of a Hydrogen plasma, radicals or Hydrogen thermal anneal. 18.The method of claim 15 wherein said threshold thickness is sufficientlysmall to permit removal of Nitrogen through said deposition layer by aHydrogen plasma, radicals or Hydrogen thermal anneal.